Mass spectrometer



Dec. 6, 1955 s 2,726,335

MASS SPECTROMETER Filed April 12, 1954 2 Sheets-Sheet 1 DETECTOR INVENTOR.

J. E. ISKE "HMAM gm ATTORNEYS J. E. ISKE MASS SPECTROMETER 2 Sheets-Sheet 2 aww INVENTOR. J E ISKE H lon f ATTORNEYS Dec. 6, 1955 Film! April 12, 1954 United States Patent 1 2,726,335 MASS SPECTROMETER James E. Iske, Bartlesville, 0kla., assignor to Phillips Petroleum Company, a corporation of Delaware Application April 12, 1954, Serial No. 422,588 15 Claims. (Cl. 25041.9)

This invention relates to mass spectrometers. In one specific aspect it relates to ion pulsing means to control the introduction of ions into a mass separating means.

Mass spectrometry comprises, in general, ionizing a sample of material under investigation and separating the resulting ions according to their masses to determine the relative abundance of ions of selected masses. The material to be analyzed usually is provided as a gas which is bombarded by a stream of electrons to produce the desired ions. Although both positive and negative ions may be formed by such electron bombardment, most mass spectrometers make use of only the positive ions. These positive ions are accelerated out of the region of the electron beam by negative electrical potentials. Such potentials impart equal kinetic energies to ions having like charges whereby ions of diiferent masses have different velocities after passing through the electrical field, and consequently have difierent momenta.

The presently known mass spectrometers can be classified into one of two general groups: the momentum selection type and the velocity selection type. The momentum selection instruments sort the ions into beams of different masses by magnetic and/or electrical deflecting fields. Ions of a selected mass are allowed to impinge upon a collector plate to which is connected a suitable indicating circuit. The velocity selection instruments sort the ions according to the velocities imparted to the individual ions by the electrical accelerating fields. The present invention is directed toward providing an improved mass spectrometer of the velocity selection type.

Mass spectrometers have recently been developed which employ a plurality of spaced grids disposed along a common axis. An alternating potential is supplied between adjacent grids. In one embodiment of this type of mass spectrometer, the grids are equally spaced in a manner analogous to a linear accelerator. The ions to be separated are accelerated into the electrical field between the first two grids. Positive ions which enter this region when the field between the first and second grids is negative will be accelerated into the space between the second and third grids. Those ions having a predetermined mass are accelerated by the first field sufiiciently to enter the second field at the instant the field between the two sets of grids is reversed. These ions are further accelerated by the second field. This accelerating process continues through the remaining fields such that ions having a predetermined mass receive maximum energy. A grid maintained at a positive potential is interposed between the last accelerating grid and the collector plate. The positive potential applied to this last grid is adjusted such that only ions which receive maximum energy by the accelerating fields pass through this last grid to impinge upon the collector plate.

A second embodiment of the velocity type mass spectrometer employs one or more groups of three grids, the individual grids in each group being equally spaced. Positive ions which enter the field between the first two grids are accelerated if this field is negative. Those ions acquiring proper velocity pass into the second electrical field at the time the electrical field reverses and are further accelerated. The ions then pass through a field-free drift space before entering a second set of similar accelerating grids. The length of this drift space is established 2,726,335 Patented Dec. '6, 1955 ice in energy while passing through the first two fields arrive at the third accelerating field at proper time to .again' be accelerated a maximum amount. One or more of these drift spaces can be provided in the mass spectrometer. A final positive grid is also disposed between the last set of accelerating grids and the collector plate to retard all ions except those which have received maximum energy.

In both of these mass spectrometers the only ions which receive maximum energy from the accelerating fields are those which enter these fields at a predetermined portion of the cycle of the alternating potential applied between adjacent accelerating grids. In the first mentioned mass spectrometer, the ions must arrive at the first accelerating grid at the time the field initially becomes negative. In the second mentioned mass spectrometer, it can be shown that the ions which receive maximum energy enter the field between the first two accelerating grids at such time as the potential on the first grid has advanced by approximately 46, assuming a sinusoidal accelerating potential is employed. In accordance with the present invention, means are provided to control the introduction of ions into the accelerating fields such that ions enter the fields only at the proper time to receive maximum energy from the accelerating fields. This is accomplished by positioning a control grid between the ion source and the first accelerating grid. This control grid normally is maintained at a positive potential of suflicient magnitude to prevent ions from passing into the accelerating fields.

However, this retarding potential is decreased momentarily at the proper time, with respect to the phase of the accelerating potential, such that a group of ions is transmitted into the accelerating fields at exactly the proper instant. The control gird thus acts as a gate which is opened momentarily to allow the ions to enter the mass separating means. The purpose of such a control grid is to reject those ions of the same mass as the desired ions being measured, but which normally would enter the grid accelerating system at times other than the time at which they receive maximum energy. Such ions, as well as those of different masses, tend to form space charges within the mass spectrometer which interfere with the proper operation of the tube. In accordance with the present invention, such ions are not allowed to enter the mass separating means and the resolution of the mass spectrometer is improved considerably.

Accordingly, it is an object of this invention to provide an improved ion velocity selection mass spectrometer.

Another object is to provide means to control the introduction of ions into amass separating system.

Various other objects, advantages and features of this invention should become apparent from the following detailed description taken in conjunction with the accompanying drawing in which:

Figure l is a schematic representation of a first embodiment of the mass spectrometer of this invention;

Figure 2 is a graphical representation of the operation of the accelerating fields in the mass spectrometer of Figure l; and

Figure 3 is a schematic. representation of a second embodiment of the mass spectrometer of this invention.

Referring now to the drawing in detail and to- Figure l in particular, there is shown a mass spectrometer tube 10 which comprises a glass envelope 11, the interior of which is maintained at a reduced pressure by a vacuum pump, not shown, which communicates with tube 10 through a conduit 12. A sample of gas to be analyzed is introduced into tube 10 through a conduit 13. An electron emitting filament 14 is disposed within one end of tube 10 and is connected to a source of electrical energy, which can be an alternating current generator 15, for example. The endterminals of a potentiometer 16 are connected to the respective end terminals of filament 14. Electrons emitted from filament 14 are accelerated into an ionization chamber 18, which is defined by a pair of grounded spaced grids 19 and 20, by an accelerating grid 21. The positive terminal of a voltage source 22 is connected to accelerating grid 21, the negative terminal of voltage source 22 being connected to the grounded center tap of potentiometer 16.

The gas molecules which enter chamber 18 through conduit 13 are bombarded by electrons emitted from filament 14. The resulting positive ions are accelerated toward a collector plate 25 mounted in the opposite end of tube by an accelerating grid 26 which is connected to the positive terminal of an adjustable voltage source 27 through a transformer winding 28. The negative terminal of voltage source 27 is connected to ground. The ions accelerated toward collector plate by grid 26 pass through three equally spaced accelerating grids 30, 31 and 32. From grid 32 the ions pass through a drift space 33 and then through three equally spaced accelerating grids 35, 36 and 37. From grid 37 the ions pass through a second drift space 38 and then through a third set of equally spaced accelerating grids 40, 41 and 42. Grids 43 and 44 are disposed between grid 42 and collector plate 25. The length of drift spaces 33 and 38 in centimeters for a field-free space is represented by the equation:

(0.659-j-2.66n)x where n is an integral number and x is the spacing between any adjacent pair of grids 30, 31, 32 or 35, 36, 37 or 40, 41, 42. Grids 31, 36 and 41 are electrically connected to one another and to one terminal of a source of alternating potential 50. Grids 30, 35, and 42 are connected to a potential dividing network 51 at points of progressively higher potentials. Network 51 comprises a resistor 52 having one end terminal grounded and the second end terminal connected to a negative potential terminal 53. Grid 32 is connected to grid 35, and grid 37 is connected to grid 40. The second terminal of voltage source is conencted to grid 30. Collector plate 25 is connected to a detector circuit 54 which measures the ion flow.

The positive ions produced within chamber 18 are accelerated toward collector plate 25 by the negative potential applied to grid 30. During one half cycle of the output signal from voltage source 50 the electrical field between grids 30 and 31 is of such phase that the ions entering this field are accelerated. Ions which enter this field at a particular phase are accelerated by a maximum amount and thereby receive maximum energy. During the next half cycle of the signal from voltage source 50 the field between grids 31 and 32 is of such phase that the ions are further accelerated. These ions then drift through the field-free space 33. The masses of the individual ions obviously influence their times of arrival at grid 35. The ions which arrive at grid 35 at the proper time are again accelerated by the field applied between grids 35 and 36 and thus receive additional energy. The same accelerating procedure continues as the ions pass through the next five grids. Thus, the resulting ion beam is velocity modulated whereby ions of a particular mass receive maximum energy. Grid 43 is connected to the terminal 55 which is maintained at a sufficient positive potential to repel all ions except those having energy greater than a predetermined value. Grid 44 is connected to a negative potential terminal 56 to suppress any secondary electrons which may be emitted within tube 19. Grid 44 serves another useful function in that it accelerates the positive ions, which pass through grid 43, into collector plate 25.

The mass spectrometer thus far described employs the principles set forth in U. S. Patent 2,535,032. As therein described, ions which are to be accelerated by a maximum amount in passing through the accelerating grids arrive at grid 30 at such time as the electrical field applied between grids 30 and 31 is of predetermined phase. This is illustrated in Figure 2 wherein the potential applied to grid 31 with respect to the potential applied to grid 30 is plotted as a function of the distance between grids 30, 31 and 32. In order for a particular ion to be accelerated a maximum amount, it is essential that the ion arrive at grid 30 at such time as the potential on grid 31, taken with respect to the potential on grid 30, has advanced approximately 46 from zero. This particular ion must arrive at grid 31 at such time as voltage curve 57 is at 180 and must arrive at grid 32 at such time as voltage curve 58 is approximately 314. An ion with such an arrival time at these grids is accelerated a maximum amount. In accordance with the present invention, grid 26 normally is maintained at a positive potential of sufiicient magnitude that no ions pass therethrough. However, voltage pulses are applied momentarily to transformer winding 28 which reduce this positive potential on grid 26 such that groups of ions are accelerated toward collector plate 25. The time at which these pulses are applied to grid 26 is adjusted such that ions of a particular mass arrive at grid 30 in proper phase, with respect to voltage source 50, to receive maximum energy.

The electrical pulsing network employed to perform this function is illustrated in Figure 1. One end terminal of voltage source 50 is connected to one end terminal of the primary winding 60 of a transformer 61. The second end terminal of transformer winding 60 is connected to one end terminal of the primary winding 62 of a second transformer 63. The second end terminal of transformer winding 62 is connected to the second terminal of voltage source 50. A variable capacitor-65 is connected in parallel with the secondary winding 66 of transformer 61 to form a first tuned circuit 58, and a variable capacitor 67 is connected in parallel with the secondary winding 68 of transformer 63 to form a second tuned circuit 59. Corresponding first end terminals of transformer windings 66 and 68 are grounded and corresponding second end terminals of these windings are connected to the respective control grids of tetrodes 70 and 71. Capacitors 65 and 67 are adjusted such that the output signals from the two tuned circuits 58 and 59 are 90 out of phase with one another. The anode of tetrode 70 is connected to a source of positive potential 72 through a first pair of series connected windings 73 and 74 of a phase shifting transformer 75 of the Drysdale type. The anode of tetrode 71 is connected to voltage source 72 through the second pair of series connected windings 76 and 77 of transformer 75. The cathode of tetrode 70 is connected to ground through a resistor 80 which is shunted by a capacitor 81, and the cathode of tetrode 71 is connected to ground through a resistor 82 which is shunted by a capacitor 83. The screen grid of tetrode 70 is connected to the contactor of a potentiometer 84 and to ground through a capacitor 85. The screen grid of tetrode 71 is connected to the contactor of a potentiometer 86 and to ground through a capacitor 87. Corresponding first end terminals of potentiometers 84 and 86 are connected to voltage source 7-2, and corresponding second end terminals of these potentiometers are connected to ground. A variable capacitor 88 is connected in shunt with series connected transformer windings 73 and 74, and a variable capacitor 89 is connected in shunt with series connected transformer windings 76 and 77.

The end terminals of the rotatable winding 91 of transformer 75 are connected to the respective end terminals of the primary winding 93 of a transformer 94. The variable capacitor 95 is connected in parallel with the secondary winding 96 of transformer 94 to form a tuned circuit 97, one terminal of which is connected to ground. The second terminal of tuned circuit 97 is connected to the control grid of a tetrode 98. The cathode of tetrode 98 is connected to ground through a resistor 99. The

.anode of tetrode 98 is connected to the control grid of atriode 100 through a capacitor 101 and to one terminal of a tuned circuit 103 which comprises an inductor 104 connected in shunt with a variable capacitor 105. The second terminal of tuned circuit 103 is connected to potential terminal 72. The screen grid of tetrode 98 is connected directly to potential terminal 72 and to ground through a capacitor 106. A resistor 107 is connected between the control grid of triode 100 and ground. The anode of triode 100 is connected to positive potential terminal 72 through a resistor 108 and to ground through a capacitor 109. The cathode of triode 100 is connected to ground through a resistor 110 and to the control grid of a pentode 112 through a capacitor 113 and a transformer winding 114, the latter two elements being connected in series relation. Transformer winding 114 constitutes one of the three windings of a transformer 115.

The cathode of pentode 112 is connected to groundv through a resistor 117. The screen grid of pentode 112 is connected to the anode thereof and the anode is connected to potential terminal 72 through a second winding 118 of transformer 114 and a resistor 119. The junction between transformer winding 118 and resistor 119 is connected to ground through a capacitor 120. The suppressor grid of pentode 112 is connected directly to ground. One end terminal of the third winding 121 of transformer 115 is connected to ground and the second terminal of transformer winding 121 is connected to the control grid of a triode 123 through a resistor 124. The control grid of triode 123 is connected to ground through a resistor 125. The cathode of triode 123 is connected to ground through a resistor 126, and the anode of triode 123 is connected to potential terminal 72 through a resistor 127. The anode of triode 123 is also connected to ground through a capacitor 128. The cathode of triode 123 is also connected to the control grid of a pentode 130 through a capacitor 131 and a transformer winding 132 which are connected in series relation. Transformer windings 132 and 28 constitute two of the three windings of a transformer 134. The junction between capacitor 131 and transformer winding 132 is connected to a negative potential terminal 116 through a resistor 135. The cathode of pentode 130 is connected to ground through a resistor 136, and the suppressor grid of pentode 130 is connected directly to ground. The screen grid of pentode 130 is connected to the anode thereof and the anode is connected to potential terminal 72 through the third winding 138 of transformer 134 and a resistor 139. The junction between transformer winding 138 and resistor 139 is connected to ground through a capacitor 140.

The output signal from voltage source 50 is applied to transformers 61 and 63, the secondaries of which provide output voltages in phase quadrature with each other. These output voltages are applied through respective isolation amplifiers 70 and 71 to the stator windings of the phase shifting transformer 75. The phase of the voltage induced in winding 91 can be adjusted through 360 by rotation of this winding. Potentiometers 84 and 86 are provided so that the magnitude of the. voltages applied to the transformer windings can be adjusted to the desired level. The voltage induced in transformer winding 91 is amplified by tetrode 98 and applied to the cathode follower circuit of triode 100. The bias potential on tube 100 is adjusted such that current pulses corresponding to the positive peaks of the grid excitation signal applied to triode 100 flow in cathode load resistor 110. These current pulses produce Voltage pulses which trigger the first blocking oscillator 112. The output pulses of oscillator 112 pass through the second cathode follower 123 and are applied to the second blocking oscillator 130 which generates pulses of extremely short duration. The output negative pulses from pentode 130 are applied to grid 26 of tube to overcome the original positive potential so that positive ions can flow through grid 26. By rotation of winding. 91, pulses of proper phase can be applied to grid 26 whereby ions transmitted therethrough arrive at grid 30 at the proper time to receive maximum energy. The position of coil 91 can be calculated from the mass of the ions to be detected, the physical dimensions of tube 10 and the magnitude of the accelerating potentials, or merely by rotating the winding until the desired output signal is obtained at detector 54.

In Figure 3 there is shown a second mass spectrometer tube which is similar in many respects to tube 10 and wherein corresponding elements are designated by like reference numerals. Tube 150 differs from tube 10 in that a plurality of equally spaced grids 151, 152, 153, 154, 155, 156, 157, 158 and 159 is employed in place of grids 30, 31, 32, 35, 36, 37, 40, 41 and 42. Grids 151, 153, 155, 157 and 159 are connected to one another and to one terminal of voltage source 50. Grids 152, 154, 156 and 158 are connected to one another and to the second terminal of voltage source 50. Tube 150 functions in somewhat the same manner as a linear accelerator in that an ion which enters the field between grids 151 and 152 is accelerated if this field is negative. If the ion is of such mass as to enter the field between grids 152 and 153 during the following half cycle of the signal from source 50, then the ion is accelerated further. Ions of a predetermined mass tend to pass through each successive field during one half cycle of the applied voltage and thus acquire maximum energy which is sufficient to penetrate'the potential barrier at grid 41. It should be apparent, however, that ions of proper mass, but which do not arrive at grid 151 at the beginning of the negative half cycle, are not accelerated sufliciently to pass grid 41'. Grid 26 is maintained at a normally positive potential to prevent the passage of ions into the accelerating region of tube 150. Negative pulses applied to grid 26 at the proper time, however, result in ions being admitted into the accelerating region. The electrical circuitry employed to provide these pulses is somewhat similar to that shown in Figure 1.

One output terminal of tuned circuit 97 of Figure 3 is applied to the control grid of a tetrode 161. The second terminal of tuned circuit 97' is connected to the negative terminal of a variable voltage source 162, the positive terminal of voltage source 162 being connected to ground. The cathode of tetrode 161 is connected to ground and the anode of tetrode 161 is connected to positive potential terminal 72' through the primary winding 163 of a transformer 164. Transformer winding 28' constitutes the secondary winding of transformer 162. The screen grid of tetrode 161 is connected to the contactor of a potentiometer 166. One end terminal of potentiometer 166 is connected to potential terminal 72' and the second end terminal of potentiometer 166 is grounded. A capacitor 167 is connected between the contactor of potentiometer 166 and ground.

Tube 161 is biased such that a negative output pulse is provided for the peak of each positive pulse applied thereto. This negative output pulse is applied to grid 26 through transformer 164 to enable ions to pass through grid 26' at the proper time to arrive at grid 151 at the beginning of the negative half cycle of voltage applied between grids 151 and 152.

In view of the foregoing description it should be apparent that there is provided in accordance with this invention an improved mass spectrometer of the velocity selection type wherein'ions are admitted into the mass separation system only at such times as ions of a predetermined mass will receive maximum energy from the separating means. This eliminates the accumulation of undesired ions in the separating means which tend to form space charges. The various grids employed in the tubes. can be of any desired open construction so that ions and electrons pass readily therethrough. These grids are referred to in the claims as ion permeable electrodes. While two forms of pulse generating circuits have been described in conjunction, with two forms of mass spectrometer tubes, it should be apparent that these. two circuits can be interchanged if desired. Furthermore, the

principles of this invention are not restricted to the particular pulse generating circuits illustrated since any C11- cuit capable of providing pulses of extremely short duration can be employed. Thus, while the invention has been described in conjunction with present preferred embodiments, it should be apparent that the invention is not limited thereto.

What is claimed is:

1. A mass spectrometer comprising, in combination, an ion source, ion detecting means, first and second ion permeable electrodes disposed between said source and said detecting means, a source of alternating potential applied between said first and second electrodes, a third ion permeable electrode disposed between said source and 'said first electrode, and means to apply potential pulses to said third electrode of predetermined phase with respect to the phase of the alternating potential applied between said first and second electrodes.

2. A mass spectrometer comprising, in combination,

an ion source, ion detecting means, first and second ion permeable electrodes disposed between said source and said detecting means, a source of alternating potential applied between said first and second elctrodes, a third ion permeable electrode disposed between said source and said first electrode, phase shifting means connected to said source of alternating potential to provide an output signal of predetermined phase with respect to the phase of said source of alternating potential, pulse forming means energized by the output signal from said phase shifting means, and means to apply the output pulses from said pulse forming means to said third electrode.

3. A mass spectrometer comprising, in combination, an ion source, ion detecting means, first and second ion permeable electrodes disposed between said source and said detecting means, a source of alternating potential applied between said first and second electrodes, a third ion permeable electrode disposed between said source and said first electrode, means for applying a bias potential to said third electrode of polarity opposite the polarity of the ions to be analyzed, phase shifting means connected to said source of alternating potential to provide an output signal of predetermined phase with respect to the phase of said source of alternating potential, pulse forming means energized by the output signal from said phase shifting means, and means to apply the output pulses from said pulse forming means to said third electrode, said pulses reducing the magnitude of said bias potential to permit ions to pass through said third electrode.

4. A mass spectrometer comprising, in combination; an ion source; ion detecting means; first, second and third ion permeable electrodes disposed between said ion source and said detecting means in the order named, the spacing between said first and second electrodes being substantially equal to the spacing between said second and third electrodes; a source of alternating potential,

one terminal thereof being connected to said second electrode and the second terminal thereof being connected to said first and third electrodes; a fourth ion permeable electrode positioned between said ion source and said first electrode; phase shifting means connected to said source of alternating potential to provide an output signal of predetermined phase with respect to the phase of said source of alternating potential; pulse forming means energized by the output signal from said phase shifting means for applying a bias potential to said fourth electrode of polarity opposite the polarity of the ions to be analyzed; and means to apply the output pulses from said pulse forming means to said fourth electrode, said pulses reducing the magnitude of said bias potential to permit ions to pass through said fourth electrode.

5. A mass spectrometer comprising, in combination; an ion source; ion detecting means; first, second, third, fourth, fifth and sixth ion permeable electrodes disposed between said ion source and said detecting means in the 'order named, the spacings between said first and second electrodes, said second and third electrodes, said fourth and fifth electrodes, and said fifth and sixth electrodes being equal, the spacing between said third and fourth electrodes being substantially (0.66+2.66n) times the distance between said first and second electrodes, 11 being an integral number; a source of alternating potential, one terminal thereof being connected to said second and fifth electrodes and the second terminal thereof being connected to said first, third, fourth and sixth electrodes; a seventh ion permeable electrode positioned between said ion source and said first electrode; phase shifting means connected to said source of alternating potential to provide an output signal of predetermined phase with respect to the phase of said source of alternating potential; pulse forming means energized by the output signal from said phase shifting means; means for applying a bias potential to said seventh electrode of polarity opposite the polarity of the ions to be analyzed; and means to apply the output pulses from said pulse forming means to said seventh electrode, said pulses reducing the magnitude of said bias potential to permit ions to pass through said seventh electrode.

6. The combination in accordance with claim 5 wherein said source of alternating potential is sinusoidal and the pulses applied to said seventh electrode are of such phase with respect to the phase of said source of alternating potential that ions of predetermined mass to be detected enter said first electrode at such time as the phase of the potential on said second electrode with respect to the potential on said first electrode is advanced approximately 46 from zero in the polarity direction opposite the polarity of the ions being detected.

7. A mass spectrometer comprising, in combination; an ion source; ion detecting means; a plurality of substantially equally spaced first ion permeable electrodes disposed between said ion source and said detecting means; a source of alternating potential, the terminals of said source of alternating potential being connected to adjacent ones of said electrodes; a second ion permeable electrode positioned between said ion source and said first electrode; phase shifting means connected to said source of alternating potential to provide an output signal of predetermined phase with respect to the phase of said source of alternating potential; pulse forming means energized by the output signal from said phase shifting means; means for applying a bias potential to said second electrode of polarity opposite the polarity of the ions to be analyzed; and means to apply the output pulses from said pulse forming means to said second electrode, said pulses reducing the magnitude of said bias potential to permit ions to pass through said second electrode.

8. A mass spectrometer comprising, in combination, an ion source, ion detecting means, first and second ion permeable electrodes disposed between said source and said detecting means, a source of alternating potential applied between said first and second electrodes, a third ion permeable electrode disposed between said source and said first electrode, first and second transformers having their primary windings energized by said source of alternating potential, the outputs of said transformers being tuned 90 out of phase with one another, a third transformer having first and second stationary windings at right angles to one another and a rotatable winding, means to apply the output voltages from said first and second transformers across said first and second windings, respectively, pulse forming means, means to apply the voltage induced in said rotatable winding to the input of said pulsc forming means, and means to apply the output pulses from said pulse forming means to said third electrode.

9. The combination in accordance with claim 8 wherein said pulse forming means comprises a vacuum tube having at least an anode, a cathode and a control grid,

means to apply the output signal from said third transformer to the grid of said tube, and means to bias said grid at a potential whereby said tube conducts during only a portion of a cycle of the output signal from said transformer.

10. The combination in accordance with claim 8 wherein said pulse forming means comprises a pair of blocking oscillators, means for applying the output signal from said third transformer to the input of the first of said oscillators, and means for applying the output signal from said first oscillators to the input of the second of said oscillators.

11. A mass spectrometer comprising, in combination; an envelope containing an electron emitting filament, a collector plate spaced from said filament, and an ionization chamber between said filament and said collector plate; means to introduce a sample of material to be analyzed into said ionization chamber; means to accelerate electrons from said filament into said ionization chamber; means to maintain the interior of said envelope at a reduced pressure; first, second, third, fourth, fifth and sixth ion permeable electrodes disposed between said ionization chamber and said collector plate in the order named, the spacings between said first and second electrodes, said second and third electrodes, said fourth and fifth electrodes, and said fifth and sixth electrodes being equal, the spacing between said third and fourth electrodes being substantially (O.66+2.66n) times the distance between said first and second electrodes, n being an integral number; a source of alternating potential, one terminal thereof being connected to said second and fifth electrodes and the second terminal thereof being connected to said first, third, fourth and sixth electrodes; means to accelerate ions from said ionization chamber toward said first electrode; a seventh ion permeable electrode positioned between said ionization chamber and said first electrode; phase shifting means connected to said source of alternating potential to provide an output signal of predetermined phase with respect to the phase of said source of alternating potential; pulse forming means energized by the output signal from said phase shifting means; means for applying a bias potential to said seventh electrode of polarity opposite the polarity of the ions to be analyzed, the magnitude of said bias potential being sufficient to prevent ions from passing between said ionization chamber and said first electrode; and means to apply the output pulses from said pulse forming means to said seventh electrode, said pulses reducing the magnitude of said bias potential to permit ions to pass between said ionization chamber and said first electrode.

12. A mass spectrometer comprising, in combination; an envelope containing an electron emitting filament, a collector plate spaced from said filament, and an ionization chamber between said filament and said collector plate; means to introduce a sample of material to be analyzed into said ionization chamber; means to accelerate electrons from said filament into said ionization chamber; means to maintain the interior of said envelope at a reduced pressure; first, second, third, fourth, fifth and sixth ion permeable electrodes disposed between said ionization chamber and said collector plate in the order named, the spacings between said first and second electrodes, said second and third electrodes, said fourth and fifth electrodes, and said fifth and sixth electrodes being equal, the spacing between said third and fourth electrodes being substantially (0.66+2.66n) times the distance between said first and second electrodes, 11 being an integral number; a source of alternating potential, one terminal thereof being connected to said second and fifth electrodes and the second terminal thereof being connected to said first, third, fourth and sixth electrodes; means to accelerate ions from said ionization chamber toward said first electrode; a seventh ion permeable electrode positioned between said ionization chamber and said first electrode; first and second transformers having their primary windings energized by said source of alternating potential, the outputs of said transformers being tuned out of phase with one another; a third transformer having first and second stationary windings at right angles to one another and a rotatable Winding; means to apply the output volatges from said first and second transformers across said first and second windings, respectively; pulse forming means; means to apply the voltage induced in said rotatable winding to the input of said pulse forming means; and means to apply the output pulses from said pulse forming means to said seventh electrode.

13. The combination in accordance with claim 12 wherein said pulse forming means comprises a vacuum tube having at least an anode, a cathode and a control grid, means to apply the output signal from said third transformer to the grid of said tube, and means to bias said grid at a potential whereby said tube conducts during only a portion of a cycle of the output signal from said transformer.

14. A mass spectrometer comprising, in combination; an envelope containing an electron emitting filament, a collector plate spaced from said filament, and an ionization chamber between said filament and said collector plate; means to introduce a sample of material to be analyzed into said ionization chamber; means to accelerate electrons from said filament into said ionization chamber; means to maintain the interior of said envelope at a reduced pressure; a plurality of substantially equally spaced first ion permeable electrodes disposed between said ionization chamber and said collector plate; a source of alternating potential, the respective terminals of said source of alternating potential being connected to adjacent ones of said electrodes; a second ion permeable electrode positioned between said ionization chamber and said first electrodes; phase shifting means connected to said source of alternating potential to provide an output signal of predetermined phase with respect to the phase of said source of alternating potential; pulse forming means energized by the output signal from said phase shifting means; means for applying a bias potential to said second electrode of polarity opposite the polarity of the ions to be analyzed, the magnitude of said bias potential being sufficient to prevent ions from passing between said ionization chamber and said collector plate; and means to apply the output pulses from said pulse forming means to said second electrode, said pulses reducing the magnitude of said bias potential to permit ions to pass from said ionization chamber to said first electrodes.

15. A mass spectrometer comprising, in combination; an envelope containing an electron emitting filament, a collector plate spaced from said filament, and an ionization chamber between said filament and said collector plate; means to introduce a sample of material to be analyzed into said ionization chamber; means to accelerate electrons from said filament into said ionization chamber; means to maintain the interior of said envelope at a reduced pressure; a plurality of substantially equally spaced first ion permeable electrodes disposed between said ionization chamber and said collector plate; a source of alternating potential, the respective terminals of said source of alternating potential being connected to adjacent ones of said electrodes; a second ion permeable electrode positioned between said ionization chamber and said first electrodes; first and second transformers having their primary windings energized by said source of alternating potential, the outputs of said transformers being tuned 90 out of phase with one another; a third transformer having first and second stationary windings at right angles to one another and a rotatable winding; means to apply the output voltages from said first and second transformers across said first and second windings, respectively; pulse forming means; means to apply the voltage induced in said rotatable winding to the input of said pulse forming means; and means to apply the output pulses from said pulse forming means to said second electrode.

No references cited. 

1. A MASS SPECTROMETER COMPRISING, IN COMBINATION, AN ION SOURCE, ION DETECTING MEANS, FIRST AND SECOND ION PERMEABLE ELECTRODES DISPOSED BETWEEN SAID SOURCE AND SAID DETECTING MEANS, A SOURCE OF ALTERNATING POTENTIAL APPLIED BETWEEN SAID FIRST AND SECOND ELECTRODES, A THIRD ION PERMEABLE ELECTRODE DISPOSED BETWEEN SAID SOURCE AND 